Low-power semiconductor memory device

ABSTRACT

A logic circuit in a system LSI is provided with a power switch so as to cut off the switch at the time of standby, reducing leakage current. At the same time, an SRAM circuit of the system LSI controls a substrate bias to reduce leakage current.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 10/274,985 filedOct. 22, 2002 (now U.S. Pat. No. 6,657,911 issued Dec. 2, 2003).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device consolidating alogic circuit and a static memory (SRAM) circuit.

2. Description of the Related Art

Japanese Published Unexamined Patent Application No. Hei 7-86916discloses a construction in which a logic circuit is provided with apower switch and a MOS transistor constructing the logic circuit isback-gate biased. Japanese Published Unexamined Patent Application No.2000-207884 discloses a substrate bias control technology for a systemLSI operable at a low voltage including a static memory. JapanesePublished Unexamined Patent Application No. 2001-93275 discloses aconstruction in which a logic circuit is provided with a logic powersource and a memory circuit is provided with a memory power source.

Currently, semiconductor integrated circuits called a system LSI (LargeScale Integrated Circuit) integrating an SRAM circuit and a logiccircuit on the same semiconductor chip are widely manufactured. The SRAMcircuit refers to a circuit functioning as a memory by only the circuitincluding arrayed SRAM memory cells and a peripheral circuit foraccessing the memory cells. The logic circuit refers to a circuitsubjecting an inputted signal to a specified process for output, otherthan the memory circuit including arrayed memory cells such as an SRAM,a dynamic memory (DRAM) and a non-volatile memory and a circuit foraccessing the memory cells. A circuit holding data such as a flip-flopin the logic circuit is thought to be part of the logic circuit.

A request for low power consumption of the system LSI and finertransistors in the LSI lower the source voltage of the LSI. In a 0.13 μmprocess, a LSI operated at a source voltage of 1.2V is manufactured.When the source voltage is lowered, the electric current of the MOStransistor is reduced to deteriorate the circuit performance. To preventthe performance from being deteriorated, a LSI lowering the thresholdvoltage of the MOS transistor is manufactured.

When the threshold value of the MOS transistor is lowered, a leakagecurrent called a sub-threshold current of the MOS transistor isincreased. The leakage current continues to flow at the time ofoperation or non-operation of the circuit. In the standby state, theSRAM does not perform write and read operations, but continues to holddata. The power consumption in the standby state of the system LSI isthe leakage current of the MOS transistor in the circuit. When thethreshold voltage of the MOS transistor is lowered, the powerconsumption in the standby state is increased. In the system LSI, thestate that the logic circuit is not operated and the SRAM circuit holdsdata is called a standby state.

The logic circuit is not operated at the time of standby. The logiccircuit cuts off the power source using a switch to reduce the leakagecurrent. The SRAM memory cell is of a flip-flop construction. The leakcurrent is relatively small. In the prior art system LSI, thecapacitance of the SRAM circuit mounted is not large and the SRAM memorycell is made by the MOS transistor having a high threshold value. Theleakage current in the SRAM circuit has not been a problem. However,when the MOS transistor is made finer and a large-capacitance SRAM ismounted on the system LSI so as to lower the threshold voltage of theMOS transistor constructing the SRAM memory cell, the leakage current inthe SRAM memory cell cannot be ignored. The logic circuit can reduce theleakage current at the time of standby when the power source is cut offby the switch. The SRAM circuit must hold data in the standby state. Thepower source cannot be cut off so that the leakage current cannot bereduced. When the voltage is lowered to reduce the threshold voltage ofthe MOS transistor, the leakage current in the circuit attached foraccessing the memory cell in the SRAM circuit is increased.

SUMMARY OF THE INVENTION

The summary of the representative inventions disclosed in thisapplication will be described as follows.

(1) In a LSI consolidating a logic circuit and an SRAM circuit, thepower source of the logic circuit is cut off by the switch at the timeof standby, and the SRAM circuit controls the substrate voltage of theMOS transistor so as to reduce the leakage current.

(2) The power source of the control circuit for accessing the memorycell in the SRAM circuit is divided to be cut off, reducing the powerconsumption.

(3) The SRAM circuit is divided so that some SRAMs hold data at the timeof standby and other SRAMs not holding data cut off the power source toreduce the leakage current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an overview of the relation between a logiccircuit and an SRAM circuit of a system LSI to which the presentinvention is applied and power sources thereof;

FIG. 2 is a schematic diagram of the layout of the system LSI of FIG. 1;

FIG. 3 is a diagram showing change in voltage of nodes in the circuitshown in FIG. 1;

FIG. 4 is a diagram showing an example of a control circuit CNTS in FIG.1;

FIG. 5 is a waveform chart of signals for changing the state of thecircuit of FIG. 1;

FIG. 6 is a diagram showing an example of a circuit generating signalsshown in FIG. 5;

FIG. 7 is a diagram showing the relation between the internalconstruction of the SRAM circuit to which the present invention isapplied and power sources thereof;

FIG. 8 is a diagram showing the construction of the logic circuit towhich the present invention is applied;

FIG. 9 is a diagram showing the constructions of transistors of thesystem LSI to which the present invention is applied;

FIG. 10 is a diagram showing a first modification of the switches of thelogic circuit;

FIG. 11 is a diagram showing change in voltage of nodes in the circuitshown in FIG. 10;

FIG. 12 is a diagram showing a second modification of the switches ofthe logic-circuit;

FIG. 13 is a diagram showing a third modification of the switches of thelogic circuit;

FIG. 14 is a diagram of an application of FIG. 13;

FIG. 15 is a diagram in which a voltage dropping circuit is applied tothe SRAM circuit;

FIG. 16 is a diagram showing change in voltage of nodes in the circuitshown in FIG. 15;

FIG. 17 is a circuit diagram of a switch circuit CNTV1 in FIG. 16;

FIG. 18 is a first modification of the SRAM circuit of FIG. 7;

FIG. 19 is a second modification of the SRAM circuit of FIG. 7;

FIG. 20 is a diagram in which substrate bias control is applied to thelogic circuit;

FIG. 21 is a diagram showing a first modification dividing the SRAMcircuit part;

FIG. 22 is a diagram showing the constructions of transistorsconstructing a plurality of SRAM circuits in FIG. 21;

FIG. 23 is a diagram showing an application of the system of theinvention of FIG. 21;

FIG. 24 is a diagram showing a second modification dividing the SRAMcircuit;

FIG. 25 is a diagram showing a third modification dividing the SRAMcircuit; and

FIG. 26 is a diagram showing a circuit construction example of a powersource control circuit CNVT2 in FIG. 24.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Embodiment 1>

FIG. 1 schematically shows the entire construction of a LSIconsolidating a logic circuit and an SRAM circuit using the presentinvention. In FIG. 1, a CHIP as the consolidated LSI includes aninput/output circuit IO (IO circuit) having power voltage lines VssQ andVddQ from outside as operating voltages; a logic circuit LOGIC executinga predetermined process to data; a static memory circuit SRAM storingdata; an nMOS transistor N1 as a switch between a ground voltage lineVss and an operating voltage supply line Vssl on the low voltage side ofthe logic circuit; a control circuit CNTS to which a signal stbyinputted in the standby state is inputted to be connected to the gateelectrode of the N1 outputting a signal cntn controlling the N1; and asubstrate bias control circuit VBBC controlling substrate voltages Vbnand Vbp of the SRAM when the stby is inputted. Unless otherwisespecified, the power source with a symbol starting from Vdd is a powersource supplying a high voltage, and the power source with a symbolstarting from Vss is a power source supplying a low voltage. Anoperating voltage difference (VssQ−VddQ) supplied to the IO circuit istypically defined by the standard and is larger than an operatingvoltage difference (Vss−Vdd) of the logic circuit and the SRAM circuit.By way of example, 3.3V is supplied to the VddQ, 1.2V is supplied to theVdd, and 0V is supplied to the Vss and the VssQ. The signal stby for usein the control circuit CNTS is used via the IO circuit.

FIG. 2 shows the layout of the circuit of FIG. 1 on a semiconductorchip. A core circuit (the logic circuits and the SRAM circuit) isarranged so as to be surrounded by the input/output circuit IO (IOcircuit). The IO circuit is connected to an input/output pad. AMOStransistor having a gate-insulating film thickness larger than that ofthe MOS transistor for use in the core circuit is used for the IOcircuit. Since a source voltage higher than that of the core circuit istypically applied to the IO circuit, the voltage resistance is required.The power switch, the substrate bias control circuit VBBC, and the powerswitch control circuit CNTS included in a power source control systemPOW in FIG. 1 are integrally arranged to increase the integrationdegree. It is advantageous when the transistor size (the channel lengthand the channel width) is different from that of the logic circuit andthe SRAM circuit. The substrate bias control circuit has a controlswitch and a charge pump circuit.

The nMOS transistor having a large insulating film thickness for use inthe IO circuit is used for the N1 in FIG. 1. The substrate voltage ofthe power switch N1 is connected to the source side. Unless otherwisespecified, the substrate voltages of the N channel type and the Pchannel type MOS transistors constructing the switch connected to thepower source are connected to the source voltages of the transistors.Use of the thick-film MOS transistor is effective for the tunnel leakagecurrent prevention of the gate. Since the voltage resistance isexcellent, the voltage applied to the gate of the switch N1 can belarger than the operating voltage and the leakage current when the nMOSis allowed to be nonconductive can be reduced. When there is only onefilm thickness of the transistor constructing the LSI and the MOStransistor for use in the IO circuit cannot be used in the core part fordesign, a MOS transistor having a small insulating film thickness can beused as a switch. In this case, the leakage current cannot be cut offcompletely by the switch N1. When the leak current is within anallowable range, the MOS switch may be only made by the thin-film MOStransistor. When the leakage current exceeds the allowable value, thesubstrate voltage of the logic circuit and the switch N1 or only theswitch N1 must be controlled to reduce the leakage current.

The nMOS transistor is used as a switch cutting off the power source ThenMOS has a flowing current larger than that of the pMOS. When attemptingto flow the same current, the switch size can be reduced. When theswitch size is not considered due to enough area, the switch of the pMOScutting off the power source Vdd can be inserted without inserting theswitch of the nMOS cutting off the ground power source Vss.

FIG. 3 shows an example of the voltages of an active state ACT and astandby state STB of the parts of the circuit. The active state ACTrepresents a state that the logic circuit and the SRAM circuit areoperated. The Vdd and the Vss in FIG. 1 are power sources of the coreincluding the SRAM circuit and the logic circuit. The Vdd voltage is1.2V and the Vss voltage is 0.0V. The standby signal stby is low in theactive state. The control signal of the switch is high and the nMOSswitch is on. The Vbn and the Vbp as the substrate voltages of the nMOStransistor and the pMOS transistor of the SRAM circuit are 0V and 1.2V.The substrate bias Vbs applied to the MOS transistor in the SRAM circuitis 0V. The threshold voltage of the MOS transistor constructing the SRAMcircuit is not changed as compared with values determined by thetransistor construction (gate width, gate length and implantationamount).

In the standby state, the standby signal stby is high. With it, thesignal cntn controlling the nMOS switch is low and the nMOS switch isnonconductive. At the same time, the Vbn and the Vbp as the substratevoltages of the nMOS transistor and the pMOS transistor constructing theSRAM circuit are −1.2V and 2.4V. A substrate bias of 1.2V is applied tothe MOS transistor in the SRAM circuit to increase the threshold voltageof the MOS transistor. The leakage current of the MOS transistor isreduced.

A circuit generating the signal cntn controlling the switch using theinputted standby signal stby can be realized by the simplified circuitas shown in FIG. 4.

When using the circuit of FIG. 4, the high standby signal stby must bealways inputted in the standby state STB, as shown in FIG. 3. Forexample, there is considered the case that the standby signal stby isinputted only in the standby state STB and an active signal ack isinputted when the standby state STB is changed to the active state ACT.FIG. 5 shows the voltage change of the standby signal stby, the activesignal ack and the control signal cntn in this case. When the standbysignal stby is inputted, the control signal cntn is low to turn off thepower switch, reducing the leakage current. When the active signal ackis inputted, the control signal cntn is high to make the power switchconductive, supplying power to the logic circuit.

FIG. 6 shows the circuit CNTS for outputting the signals of the waveformof FIG. 5. To store the standby state STB in the circuit, a flip-flop isused. In this case, a signal for returning to the active state ACT isprepared.

FIG. 7 shows a construction example of the SRAM circuit SRAM in FIG. 1.The SRAM circuit includes a memory cell array MAR, peripheral circuitsPERI1 and PERI2 for accessing the memory cells, and an inverter to inputan inverted signal of the signal stby transmitting the standby state tothe gates of MOS transistor s_sw2 and s_sw1 and s_sw2 m as switches forcutting off the power lines Vss and Vdd of the PERI1 or the PERI2. Thesubstrate voltage of the P channel type MOS transistor included inSRAM_CIR is connected to the Vbp and the substrate voltage of the Nchannel type MOS transistor included therein is connected to the Vbn soas to control the substrate-bias voltage. The MAR is a circuit havingthe arrayed SRAM memory cells. The memory cell has a pair of flip-flops(a first and second P channel type load MOS transistors and a first andsecond N channel type drive MOS transistors) having the input and theoutput of a CMOS inverter interconnected, and a first and second Nchannel type transfer MOS transistors connected between two storagenodes and bitlines (BL, /BL) of the flip-flops. The gate electrode ofthe N channel type transfer MOS transistor is connected to a wordlineWL. The operating voltage of the memory cell is given by Vddma andVssma.

The peripheral circuit PERI1 includes a circuit for controlling thewordline WL of the memory cell including a word driver WDR, a rowdecoder RDEC, and a memory controller MCNT. The operating voltage of thecircuit included in the PERI1 is given by Vddper and Vssper.

The peripheral circuit PERI2 includes a circuit for controlling abitline BL of the memory cell including a pre-charge circuit PRE, aread/write amplifier RWAMP as a read/write control circuit connected tothe bitline and a column decoder CDEC. The read/write amplifier RWAMPincludes OBUF as an output buffer of a sense amplifier and WCNT as acontrol circuit of the write amplifier. The operating voltage of thecircuit included in the PERI2 is given by Vddamp and Vssamp. A lowsignal is inputted to /stby in the drawing at the time of the standbySTB. At the time of standby, the power line Vdd inputted to the PERI1 iscut off, and at the same time, the power line Vss inputted to the PERI2is cut off. The Vbn and the Vbp supplying the substrate voltage of theMOS transistor constructing the MAR, PERI1 and PERI2 are controlled soas to increase an absolute value of the threshold voltage of the MOStransistor. The substrate bias is applied to the SRAM memory cell. Thesubstrate bias is applied to the peripheral circuit. The power source isprovided with the switch to reduce the leakage current. The powerconsumption of the SRAM at the time of standby can be reduced.

There will be described next a reason why the peripheral circuit isdivided into two to cut off the Vss and the Vdd in FIG. 7. The wordlineis low in the standby state and is low also in the operating stateexcept for the selected wordline. The circuit driving the wordline cutsoff the power source Vdd as the high voltage to reduce the leakagecurrent when compared to cutting off the power source Vss as the lowvoltage. Time for returning from the standby state is shortened. Theswitch inserted in the Vdd side is smaller than that inserted in the Vssside. The bitline is typically charged to the Vdd in the SRAM. In manycases, the amplifier is of a construction in which the state charged tothe Vdd is stable. The bitline is charged to the Vdd at the time ofstandby to cut off the power source Vss of the read amplifier and thewrite amplifier by the switch The leakage current can be thus reduced.Time for returning from the standby state to the active state isshortened. In the circuit pre-charging the bitline to the Vdd, the Vssof the circuit driving the bitline is cut off, which is advantageous inthe leakage current and the return time. In the circuit pre-charging thebitline to the Vss, cutting off the switch on the Vdd side is naturallyadvantageous, providing the construction.

The circuit in FIG. 7 is assumed to be the SRAM in the system LSI asshown in FIG. 1, and can be applied to a memory LSI without beinglimited to the system LSI. FIG. 7 is a diagram controlling the substratebias of the SRAM circuit. When the leakage of the peripheral circuit canbe reduced to sufficiently reduce the power consumption in the standbystate, the substrate bias is not necessarily applied. In the case thatthe characteristic of the MOS transistor will be varied in the future tomake the leakage current called a junction leakage of the MOS transistorlarger than the leakage current called a sub-threshold of the MOStransistor, the method for controlling the substrate voltage may notreduce the leakage current. In this case, a construction in which thepower sources of the logic circuit and the peripheral circuits of theSRAM in the system LSI are cut off by the switches can be thought to bean important technology.

FIG. 8 shows a construction example of the logic circuit LOGIC in FIG.1. In a logic circuit LOGIC_CIR, the logic gates such as NAND and NOR asthe inverter having the P channel type MOS transistor and the N channeltype MOS transistor are combined for multiple-stage connection. Thesubstrate voltage is not applied to the transistor in the logic circuit.The substrate voltage of the P channel type MOS transistor is connectedto the high voltage side Vdd of the operating voltage and the substratevoltage of the N channel type MOS transistor is connected to the lowvoltage side Vssl of the operating voltage.

FIG. 9 shows the constructions of the types of a MOS transistor for usein the logic circuit and the SRAM circuit (CORE) in the LSI, a MOStransistor for use in the input/output circuit IO of the LSI, a MOStransistor for use in the switch logic sw cutting off the power sourceof the logic circuit shown in FIG. 1, and a MOS transistor for use inthe switch s sw cutting off the power sources of the peripheral circuitsof the SRAM shown in FIG. 7. The threshold voltages of the P channeltype MOS and the N channel type MOS transistors are different. Thepolarity is typically inverted to be designed to be the same value. FIG.16 shows its absolute value. A transistor having a large insulating filmthickness is used for the input/output circuit part of the LSI. Atransistor having a small insulating film thickness is used for theinternal logic circuit. In this drawing, there are used a MOS transistorhaving a large insulating film thickness of 6.7 nm and a MOS transistorhaving a small insulating film thickness of 2.0 nm. As the transistorhaving a small film thickness, a MOS transistor having two or morethreshold voltages depending on a difference in impurity amount is oftenused.

In FIG. 9, a MOS transistor having two threshold voltages Vth of 0.40Vand 0.25V is taken as an example. The current at the time of operationof the MOS transistor having a low threshold voltage is large and theleakage current at the time of standby is increased. In all thecombinations, the MOS transistor of a thin gate-insulating film havingtwo types of Vth is used for the logic circuit LOGIC_CIR and the SRAMcircuit SRAM_CIR except for the control switch, and the MOS transistorof a thick gate-insulating film having a high Vth is used for the IO. Inthe LOGIC_CIR, a transistor having a low threshold value is used for acritical path and a transistor having a high threshold value is used forthe remaining circuits. In the SRAM_CIR, to reduce the leakage currentand to maintain static noise margin (SNB), a transistor having a highthreshold value is used for the memory cell array MAR. Since theperipheral circuit PERI including the pre-charge circuit, the senseamplifier, the word driver and the decoder must be fast, a MOStransistor having a low threshold value is used.

In the combination of Pattern1, the thick-film MOS transistor having ahigh Vth is used for the power switch of the logic circuit, and thethin-film MOS transistor having a high Vth is used for the power switchof the peripheral circuit in the SRAM circuit. The thick-film MOStransistor is used for the power switch of the logic circuit to reducethe leakage current of the large-scale circuit. The SRAM controls thesubstrate bias to reduce the leakage current. The thin-film MOStransistor having a slightly large leakage is used for the power switchto reduce the entire leakage current.

When the circuit size of the peripheral circuit in the SRAM circuit isnot large, the leakage current of the peripheral circuit is consideredto be small. The construction of the Pattern1 is effective. Whendesigning the SRAM circuit as a module which can be easily reused, theSRAM circuit can be designed in consideration of the characteristic ofonly the thin-film MOS transistor. The switch is made by the thin-filmMOS transistor to enhance the design efficiency.

The construction of the Pattern1 is effective when the size of the SRAMcircuit itself is not large, the size of the peripheral circuit in theSRAM circuit is not large, reduction of the leakage current is highlyexpected by controlling the substrate bias, or the design efficiency ofthe SRAM is considered.

In the combination of Pattern2, the thick-film MOS transistor having ahigh Vth is used for the power switch of the logic circuit and the powerswitch of the peripheral circuit in the SRAM circuit. This can reducethe leakage current of the circuit other than the SRAM memory cell inthe LSI. The power consumption at the time of standby is smaller thanthat of the Pattern1. When designing the SRAM circuit, thecharacteristic of the thick-film MOS transistor must be considered. Thedesign efficiency is thus lowered. The combination of the Pattern2 iseffective when the size of the SRAM circuit is large, the size of theperipheral circuit of the SRAM is large, or an effect for reducing theleakage current by controlling the substrate bias is not highlyexpected.

In the combination of Pattern3, the thin-film MOS transistor having ahigh Vth is used for the power switch of the logic circuit and the powerswitch of the peripheral circuit in the SRAM circuit. In this case, thethin-film MOS transistor is used. An effect for reducing the leakagecurrent is lower than that of the Pattern2. Since the characteristic ofthe thick-film MOS transistor need not be considered, the designefficiency is enhanced. The combination of the Pattern3 is effectivewhen the effect for reducing the leakage current of the LSI is lesslarge and the design efficiency is required.

As described above, the power source of the logic circuit is cut off bythe switch in the standby state and the substrate bias is applied to theSRAM circuit. The leakage current of the system LSI is lowered to reducethe power consumption in the standby state.

<Embodiment 2>

This embodiment shows a modification of the power switch for use in thelogic circuit. FIG. 10 shows a circuit block diagram when the powerswitch mounted only on the power source Vss of the logic circuit part inthe circuit of FIG. 1 is provided in the power sources Vdd and Vss. Thetwo power sources Vdd and Vss of the logic circuit are provided with theswitch to cut off the power source. The area is increased by providingthe power switch. The leakage current at the time of standby can be cutoff more reliably. FIG. 1 illustrates the IO circuit, but is omitted inFIG. 10. The IO circuit in the CHIP is omitted in other drawings.

FIG. 11 shows the voltages of the parts of the circuit when using thecircuit of FIG. 10. In this drawing, a signal cntp controlling P1 of thepMOS as a switch for cutting off the Vdd of the logic part is added tothe voltages of FIG. 3. The cntp is low in the active state ACT and ishigh in the standby state STB. The internal circuit of a circuit CNTS2outputting the control signal in FIG. 7 is not particularly described. Acircuit outputting a phase reversal signal can be added to the circuitof FIG. 4 or FIG. 6.

In FIGS. 1 and 10, the circuit when the logic circuit in FIG. 1 isunified is described. FIG. 12 shows a block diagram of a circuit whenthe present invention is applied to the LSI in which the logic circuitis divided into two or more blocks. FIG. 12 shows an example when thelogic circuit is divided into two blocks. When it is divided into threeor more blocks, the same construction can be applied. The memoryconsolidated LSI shown in FIG. 12 has logic circuits LOGIC1 and LOGIC2,ground voltage power lines Vssl1 and Vssl2 of the LOGIC1 and the LOGIC2,switches N2 and N3 connecting the Vssl1 and the Vssl2 to the power lineVss of the entire LSI, the static memory circuit SRAM, the controlcircuit CNTS of the switch, and the circuit VBBC controlling thesubstrate voltage of the SRAM. It is of the same construction as thecircuit of FIG. 1 except that there are a plurality of logic circuitsand the operation is the same as that of the circuit of FIG. 1. Thelogic circuit is divided into a plurality of blocks which are providedwith a switch cutting off the power source. An optimum switch can beadded to each of the blocks. For example, an nMOS switch cutting off theVss is added to some logic blocks and a pMOS switch cutting off the Vddis added to other blocks. Otherwise, a switch cutting off the two powersources Vdd and Vss can be provided depending on the block.

In the memory consolidated LSI shown in FIG. 13, the power switch isadded to each of the blocks of the logic circuit so that the powerswitch is controlled by other signals cntn1 and cntn2. The LSI has acontrol circuit CNTS3 which can separately control the control signalscntn1 and cntn2. These points are different from FIG. 12. The CNTS3 is acircuit which can control the control signals cntn1 and cntn2 of thepower switch and permits control so that the switch N2 is cut off andthe switch N3 is conductive depending on the operating state of thecircuit. Thus, the block of the logic circuit which must be operated inthe standby state is operated and the block of the logic circuit and theSRAM circuit which can be stopped are brought into the standby state,reducing the leakage current.

In FIG. 13, as in the case of FIG. 12, when there are three or morelogic blocks, it is possible to make a combination cutting off the powersource on the Vss side, the power source on the Vdd side or both foreach of the blocks. In the construction of FIG. 13, the power supply foreach of the blocks is controlled so as to bring the standby state, thatis, the low leakage state. Not only in the standby state but also in theactive state, the power switch is controlled so that the logic circuitand the SRAM circuit which need not be operated are brought into thestandby state, minimizing the leakage current.

FIG. 14 shows an example in which the embodiment of FIG. 13 is appliedto a system (microcomputer) equipped with a central processing unit. Thesystem LSI has a logic circuit block CPU called a central processingunit CPU permitting various arithmetic operations, a logic circuit blockDSP dedicated for digital signal operation, a static memory block SRAMcircuit, a bus BUS connecting the blocks exchanging data, a circuitBSCNT controlling the bus, and a circuit IO exchanging data withoutside. The respective blocks exchange data via the bus in the activestate. Whether the bus is operated or not is found by monitoring theoperating state of the bus. For example, when the entire circuit is notoperated, the circuit BSCNT controlling the bus transmits that all theblocks are in the standby state, to the control circuit CNTS3 of theswitch by a signal stat1. The CNTS3 makes the cntn1 and the cntn2 low tocut off the switches N2 and N3, reducing the leakage current of thelogic circuit. At the same time, the VBBC controls the Vbn and the Vbpas the substrate voltages of the SRAM to lower the leakage current ofthe SRAM, reducing the leakage current of the entire circuit.

When only the CPU is operated and there is no access to the DSP and theSRAM via the bus, the BSCNT outputs the information via the stat1 toallow the substrate voltage of the SRAM to be in the standby state, tocut off the power switch N3 of the DSP to allow the DSP to be in thestandby state, and allows only the CPU to be in the active state.

<Embodiment 3>

FIG. 15 schematically shows the entire construction of the LSIconsolidating the logic circuit and the SRAM circuit using the presentinvention. The CHIP as the consolidated LSI includes the logic circuitLOGIC; the static memory circuit SRAM; the nMOS transistor N1 as aswitch between the ground voltage line Vss from outside and the groundvoltage line Vssl of the logic circuit; the control circuit CNTS towhich the signal stby inputted in the standby state is inputted to beconnected to the gate electrode of the N1 outputting the signal cntncontrolling the N1; the substrate bias control circuit VBBC controllingthe substrate voltages Vbn and Vbp of the SRAM when the stby isinputted; and a circuit CNTV1 controlling the power line Vddm of theSRAM by the stby signal.

The construction of FIG. 15 is the same as that of the circuit of FIG. 1except for the CNTV1. Other than the CNTV1, it is operated as in FIG. 1.The CNTV1 is brought into the standby state and the stby signal isinputted. The source voltage of the SRAM is lower than the Vdd which canhold data from the Vdd. This controls the substrate voltage of the SRAMin the standby state to reduce the leakage current. Since the sourcevoltage is lowered, the leakage current can be further reduced. Thepower consumption at the time of standby can be lower than that of thecircuit of FIG. 1.

FIG. 16 shows the voltages at the time of active ACT and the time ofstandby STB of the parts of the circuit of FIG. 15. The voltage of thepower source Vdd of the circuit exhibits 1.2V. The stby, cntn, Vbn andVbp are the same as those of FIG. 3 as the operating voltages of FIG. 1.The source voltage Vddm of the SRAM is 1.2V at the time of active ACTwhich is the same as the source voltage and is 0.6V at the time ofstandby STB. This can reduce the leakage current in the SRAM.

The power source control circuit CNTV1 in FIG. 15 can be realized by thecircuit of FIG. 16. The CNTV1 has a voltage dropping circuit PDC and acircuit changing switch. When the SRAM circuit is in the active stateACT, the circuit changing switch connects the power line Vddm supplyingthe operating voltage to the memory cell in the SRAM circuit to thepower source Vdd supplied from outside so that the source voltage of theSRAM circuit is equal to the Vdd. In the standby state STB, the signalstby switches the circuit changing switch to connect Vddlow which isabove the voltage lower than the Vdd produced by the voltage droppingcircuit holding data in the memory cell of the SRAM and the power sourceVddm of the SRAM circuit. The source voltage of the SRAM circuit islower than the Vdd. In FIG. 15, the voltage is dropped on the highvoltage side. The power source control circuit CNTV1 is connectedbetween Vssm and the Vss so that the CNTV1 can be a voltage raisingcircuit. The raised voltage or a combination of the raised and droppedvoltages on the low voltage side can obtain the same effect.

<Embodiment 4>

FIG. 18 shows a modification of the circuit of FIG. 7. In FIG. 7, thepower sources of the memory cell array are the Vddma and the Vssma, thepower sources of the circuit RWAMP including the circuit driving thebitline are the Vddamp and the Vssamp, and the power sources of othercircuits are the Vddper and the Vssper. The power source in the SRAMcircuit is divided into three systems. A switch having the N channeltype MOS transistor is inserted between the peripheral circuit PERI2 foruse in control of the bitline and the power source on the low voltageside. A switch having the P channel type MOS transistor is insertedbetween the peripheral circuit PERI1 for use in control of the wordlineand the power source on the high voltage side. The switches are insertedin the high voltage side and the low voltage side of the three-systempower sources to cut off the power sources at the time of standby. Inthis circuit, the switches constructed by the MOS transistor areinserted in all the power sources. At the time of standby, controlsignals cntmp1 and cntmp2 become low, a control signal cntmp3 becomeshigh, control signals cntmn1 and cntmn3 become high, and a controlsignal cntmn2 becomes low. Switches P6, P7, N6 and N8 are conductive tocut off switches P8 and N7, realizing the construction of FIG. 7. Tohold information of the SRAM memory cell, the P6 and the N6 must beconductive at the time of standby. When the SRAM circuit is divided intoblocks, as described later, use of the construction cutting off the P6and the N6 in the block which need not hold the information is effectivein low power consumption.

Without allowing the signal cntmn2 applied to the switch controlling theVssamp at the time of standby to be low, the cntmp2 is allowed to behigh. It is possible to realize a circuit cutting off the power sourceon the Vdd side of the read/write amplifier which is considered to beused in the circuit pre-charging the bitline to be low. In the circuitshown in FIG. 18, some kinds of circuits can be realized depending onthe control method of the control signal.

FIG. 19 shows a circuit which partly modifies of the circuit of FIG. 7.The power source of the column decoder CDEC is connected to the Vddperand the Vssper. In FIG. 7, the column decoder CDEC cuts off the powersource on the Vss side. The column decoder is arranged near theamplifier. When the same switch as that of the amplifier cuts off thepower source, the circuit design is considered to be facilitated. Thepower source is shared between the column decoder and the circuitcontrolling the wordline to cut off the power source of the Vdd side.Unless the design of the power source arrangement is complicated, theVdd side can be cut off. The column decoder controls the bitline and hasmany nodes having the low voltage at the time of operation as in theword driver WDR (the number of the selection lines is larger than thatof the non-selection lines). It is advantageous to provide the sameoperating voltages Vddper and Vssper as those of the memory controllerMNCT using the switch on the high voltage side. By the same reason,although not shown, the control circuit WCNT of the write amplifier isdesirably connected to the Vddper and the Vssper.

In FIG. 19, there are prepared the switches for the three-power sourcesof FIG. 18 (the switches of the P channel type MOS transistors providedbetween the Vdd and the Vddma, and the Vddamp and the Vddper; and theswitches of the N channel type MOS transistors provided between the Vssand the Vssma, and the Vssamp and the Vssper) and the power line not viathe switch. For the construction, depending on whether the switch forcutting off is provided on the low voltage side or the high voltageside, the SRAM circuit is blocked. The P channel type MOS transistorconnected to the Vddamp and the N channel type MOS transistor connectedto the Vssper can be omitted.

In this circuit, unlike FIG. 7, the power source connected to the rowdecoder RDEC is directly connected to the power sources Vdd and Vssoutside the SRAM circuit, not to the cuttable power source in the SRAMcircuit. Power is supplied to the row decoder at the time of standby.This prevents the transfer MOS in the memory cell from being conductivedue to noise on the wordline caused by a time difference in power supplyat the time of returning from the standby state.

The noise is produced because the power source of the previous-stagecircuit of the power source of the word driver rises quickly to input alow signal to the word driver, and then, the word driver outputs a highsignal. Power is supplied to the row decoder at the time of standby. Thelow signal is not inputted to the word driver. The noise cannot beprovided on the wordline.

In FIG. 19, the power source of the entire row decoder is directlyconnected to the power sources Vdd and Vss supplied from outside. Thecircuit construction cannot reduce the leakage current of the rowdecoder. Although not shown, the power source of the entire row decoderis not connected to the Vdd and the Vss. The Vdd and the Vss areconnected only to the previous-stage circuit of the word driver, e.g.,the NAND circuit. There can be considered a circuit construction inwhich the power source cut off by the switch is connected to othercircuits of the row decoder. The circuit construction can reduce theleakage current. The power source arrangement in the row decoder iscomplicated so that the design is difficult. When the size of the rowdecoder in the SRAM circuit is relatively large and the leakage currentof the row decoder must be reduced, the power sources Vdd and Vsssupplied from outside are connected only to the previous-stage circuitof the word driver. The power source which can cut off the power sourceat the time of standby by the power switch is connected to othercircuits of the row decoders. When the size of the row decoder is notlarge and the influence of the leakage current of the row decoder isless, all the power sources of the row decoder are connected to the Vddand the Vss. This construction is considered to be effective.

As shown in FIG. 19, the power source of the peripheral circuit of theSRAM is divided by function for control to reduce the leakage current ofthe peripheral circuit of the SRAM.

<Embodiment 5>

FIG. 20 shows a construction diagram in which the LSI consolidating thelogic circuit and the SRAM circuit of FIG. 1, substrate bias control isperformed not only to the SRAM circuit but also to the logic circuit.The CHIP as the consolidated LSI includes the logic circuit LOGIC; thestatic memory circuit SRAM; the nMOS transistor N1 as a switch betweenthe power source Vss and the ground voltage line Vssl of the logiccircuit; a switch SW1 selecting, to which Vdd, Vss, Vbn and Vbp, thesubstrate voltage lines Vbn1, Vbp1, Vbnm and Vbpm of the MOS transistorconstructing the logic circuit and the SRAM circuit are connected; acontrol circuit CNTS4 outputting the signal cntn controlling the N1 andsignals cntvbb1 and cntvbb2 controlling the switch SW1; and a substratebias control circuit VBBC2 producing the substrate bias Vbn and Vbp.

The voltages of the parts at the time of active and standby are shown inFIG. 3. At the time of standby, the power source of the logic circuit iscut off The substrate voltage of the logic circuit is controlled toreduce the leakage current of the logic circuit.

When the power switch of the logic circuit is made by the MOS transistorhaving a low threshold value, as shown in the Pattern3 of FIG. 9 andthere is a leakage current in the power switch, the substrate bias isapplied to reduce the leakage current of the logic circuit. The circuitis thus effective. In the circuit, the substrate voltages of the logiccircuit and the SRAM circuit can be controlled independently. Only theSRAM circuit is allowed to be in the standby state and the logic circuitis allowed to be in the active state. When only the logic circuit isoperated, the leakage current in the SRAM circuit can be reduced.

The substrate bias is applied to the logic circuit to operate the SRAMcircuit, reducing the leakage current of the logic circuit. The switchwhich can select whether the substrate bias is applied to the logiccircuit and the SRAM circuit is provided, permitting an operation toreduce the leakage current corresponding to the operating state.

The block controlling the substrate voltage is controlled finely. Thesubstrate bias is applied to change the load amount varying the voltage.That is, when the switch is provided so as not to apply the substratebias to the unnecessary parts, the load which must change the voltage isreduced. Time to change the voltage can be shortened.

<Embodiment 6>

FIG. 21 shows a first modificatiOn in which the SRAM circuit of FIG. 1is divided into blocks. In FIG. 21, the CHIP as the consolidated LSIincludes the logic circuit LOGIC; static memory circuits SRAM1 andSRAM2; an nMOS transistor N9 as a switch between the power source Vssand the ground voltage line Vssl of the logic circuits an nMOStransistor N10 as a switch between the power source Vss and a groundvoltage line Vssm1 of the SRAM1; the control circuit CNTS outputting thesignal cntn controlling the N9 and the N10; and the substrate biascontrol circuit VBBC producing the substrate bias Vbn and Vbp. The SRAMcircuits SRAM1 and SRAM2 can be of the same construction as that of FIG.7 and the already described modification of FIG. 7.

In the circuit, the SRAM circuit of FIG. 1 is divided into two blocks ofthe SRAM1 and the SRAM2. At the time of standby, the power sources ofthe logic circuit and the SRAM1 are cut off and the substrate bias isapplied to the SRAM2 to reduce the entire leakage current, reducing thepower consumption at the time of standby. As compared with the circuitof FIG. 1, the leakage current of the circuit of the SRAM1 can bereduced. In the construction, however, data stored into the SRAM1 iserased at the time of standby. Data which must be stored at the time ofstandby must be stored into the SRAM2. In the system LSI, some SRAMblocks are consolidated so that blocks which must and need not hold dataat the time of standby are often consolidated. In such circuit, thecircuit construction is used to provide a large effect for reducing theleakage current.

FIG. 22 shows a combination of the MOS transistors for use in the memorycell circuits of the divided two SRAM circuits. In the drawing, as inFIG. 9, the MOS transistor having a large insulating film thickness of6.7 nm and the MOS transistor having a small insulating film thicknessof 2.0 nm are used. As an example in which the MOS transistor with asmall film thickness has two threshold voltages, the MOS transistorhaving two threshold voltages Vth of 0.40V and 0.25V is used. A MAR1represents the MOS transistor of the memory cell of the SRAM1 cuttingoff the power source. A MAR2 represents the MOS transistor of the memorycell of the SRAM2 not cutting off the power source. As described in thetable of FIG. 9, the MOS transistor having two threshold values is usedfor the logic circuit LOGIC_CIR. The MOS transistor having a lowthreshold value is used for about 10% in the logic circuit. They areallocated to the transistors in a critical path.

The MOS transistor of the peripheral circuit except for the memory cellof the SRAM is not shown. The MOS transistor having the same thresholdvalue as the low threshold value of 0.25V of the logic circuit is used.The thin-film MOS transistor is used in any case. The IO represents theMOS transistor for use in the input/output circuit. The thick-film MOStransistor having a high threshold voltage is used in any of thecombinations.

In a Pattern1, the thin-film MOS transistor having a high thresholdvoltage is used for the memory cell in all the SRAM circuit blocks. Theconstruction is-thought to reduce the area of the memory cell and hasthe excellent operating stability of the SRAM.

In a combination of the Pattern2, the SRAM memory cell to which thepower switch is not added is made by the thick-film MOS transistorhaving a small leakage current to reduce the leakage current. In thecombination, the transistor constructing the memory cell in the SRAM1including the power switch is made by the thin-film MOS transistor. Thearea is small and the operation is fast. The leakage current can bereduced by the power switch.

The transistor constructing the memory cell in the SRAM2 not includingthe power switch is made by the thick-film MOS transistor. The leakagecurrent at the time of standby can be reduced. The area of the memorycell of the SRAM2 is considered to be increased. The transistor is usedin the circuit in which the circuit area is not cared much and theleakage current must be reduced reliably. The highest advantage of thecircuit is thus obtained. When the circuit size of the SRAM2 is small,the combination is effective.

In a Pattern3, the memory cell of the SRAM1 is made by the thin-film MOStransistor having a low threshold voltage. The memory cell of the SRAM2is made by the thin-film MOS transistor having a high threshold voltage.When the threshold value of the MOS transistor constructing the memorycell of the SRAM is lowered, there are problems that the leakage currentis made higher to increase the power consumption at the time of standbyand the operating margin of the SRAM itself is lost so as not to operatethe memory cell itself. The former problem can be avoided by providingthe power switch. The combination can be realized only when using theMOS transistor having a characteristic so as not to exhibit the latterproblem significantly.

In a Pattern4, the thick-film MOS transistor is used as the MOStransistor constructing the SRAM2 memory cell of the combination of thePattern3. This increases the circuit area than that of the Pattern3. Theleakage current can be reduced.

FIG. 23 shows an application of FIG. 21. The CHIP as the consolidatedLSI includes the logic circuit LOGIC; the static memory circuits SRAM1and SRAM2; the BUS as a bus transferring data between the logic circuitand the SRAM circuit; an nMOS transistor N9 as a switch between thepower source Vss and the ground voltage line Vssl of the logic circuit;the nMOS transistor N10 as a switch between the power source Vss and theground voltage line Vssml of the SRAM1; a control circuit CNTS5outputting the control signals cntn and dtran in the standby state; andthe substrate bias control circuit VBBC producing the substrate bias Vbnand Vbp.

Typically, in the system LSI, data is exchanged between the logiccircuit and the SRAM circuit via the bus. The bus is considered to existin the circuit of the system LSI of FIG. 21. The circuit of FIG. 23 isdifferent from the circuit of FIG. 21 in that it has the CNTS5 as thecontrol circuit in the standby state. The operation of the circuit andthe operation of the bus will be described.

When the LSI is allowed to be in the standby state, the control circuitCNTS5 controls the logic circuit using the control signal dtran toescape data in the SRAM1 which must be stored at the time of standby tothe SRAM2 via the bus. Upon completion of the escape, the escapecompletion is transmitted to the control circuit CNTS5 via the dtran.The control circuit CNTS5 outputs a signal for change to the standbystate to cut off the power sources of the logic circuit and the SRAM1 bythe switch. Then, the substrate bias to reduce the leakage current isapplied to the SRAM2. For returning from the standby state to the activestate, the control circuit CNTS5 outputs a signal to supply power to thelogic circuit and the SRAM1, and then, the substrate bias in the standbystate of the SRAM2 is switched to the bias in the active state. When thesource voltages of the logic circuit and the SRAM1 and the substratevoltage of the SRAM2 are stable, the circuit controlling the bus via thecontrol signal dtran is controlled to restore data of the SRAM1 escapedto the SRAM2. The circuit can hold data which must be held at the timeof standby and reduce the leakage current of the memory cell of datawhich need not be held.

FIG. 24 shows the SRAM circuit of a second modification in which theSRAM circuit is divided into blocks and its power source control circuitpart. In FIG. 15, the CNTV1 controls the voltage on the high voltageside of the SRAM circuit by one voltage dropping circuit. Optimumcontrol (The voltage of the SRAM1 is dropped and the voltage of theSRAM2 is not dropped since read and write operations are performed) canbe performed for each of the blocks by dividing the SRAM circuit. As inthe case of FIG. 15, the voltage is not dropped on the high voltage sideand the raised voltage or a combination of the raised and droppedvoltages on the low voltage side can obtain the same effect. The circuitshown in FIG. 17 may be used for each CNTV2. The dropped voltage must beabove the lowest voltage which can hold the memory of the SRAM.

FIG. 25 shows the SRAM circuit of a third modification in which the SRAMcircuit is divided into blocks and its power source control circuitpart. They have four SRAM blocks SRAM1, SRAM2, SRAM3 and SRAM4, switchesP9, P10, P11 and P12 constructed by the P channel type MOS transistorcutting off the power sources of the blocks, and a control circuit CNTS6controlling the power switches. At the time of standby, the power sourceof the block which needs to hold data is not cut off and the powersource of the block which need not hold data is cut off. In the circuitconstruction, the leakage current of the SRAM circuit is produced onlyin the blocks which must hold data. An example of the P channel type MOStransistor is described. As described already, the constructionsubstituting it for the N channel type transistor is advantageous inarea efficiency.

In FIG. 21, the block not cutting off the power source is provided andinformation of another block which must hold information and cuts offthe power source at the time of standby is transferred. Thisconstruction need not transfer data. Means for detecting whetherinformation must be held or not is added. The means cuts off only thepower switch of the block which need not hold information.

There is considered a control method of a control circuit CNTS6 in whichthe block into which necessary data is stored is stored and in the caseof change to the standby state, the power source of the block in whichno data is stored is cut off. In addition, there is considered a controlmethod in which when making the circuit, the block cutting off the powersource and the block not cutting off the power source are programmed tocut off the power source corresponding to the program. Further, there isconsidered a control method in which the power source of the block to becut off during operation is programmed to cut off the power source ofonly the block which must cut off the power source. In this manner,change of the control methods of the control circuit CNTS6 can realizevarious power source cutting-off patterns.

In FIG. 26, the CNTV2 of FIG. 24 is a switch connected to any one ofthree power source states. The three power source states refer to astate connected to the source voltage Vdd supplied from outside, a stateconnected to the power source having a voltage lower than the Vdd whichcan hold data in the SRAM, and a state cutting off the power source.When the CNTV2 can be connected to the three power source states, thepower sources of all the blocks are connected to the Vdd in the activestate, the power source of the block which must hold data is connectedto the power source having a voltage lower than the Vdd in the standbystate, and the power source of the block which need not hold data is cutoff. This can reduce the leakage current of the block which must holddata. The PDC in the drawing is a voltage dropping circuit and outputs avoltage lower than the power source Vdd in which the memory cell of theSRAM can hold data. In the circuit, corresponding to the value of theinputted control signal a cntp1, the switch is switched to connect thepower source Vddm of the memory to the Vdd, the circuit dropping thevoltage of the Vdd, or a state connected to nothing.

Use of the circuit of FIG. 25 can reduce the leakage current in thestandby state. The power source of the SRAM block accessed in the activestate is connected to the Vdd and the power source of the block notaccessed is connected to the power source having a voltage lower thanthe Vdd. This can reduce the leakage current of the unnecessary SRAMblock at the time of active. The CNTV2 can also be changed from thevoltage dropping circuit to the voltage raising circuit, which can beinserted between the power line and the power source on the low voltageside of the memory cell. In FIGS. 24 and 25, the SRAM circuit is dividedinto four blocks. As the circuit construction, it can be applied whenthere are one or more blocks.

The MOS (Metal-Oxide-Semiconductor) transistor is described above. TheMIS (Metal-Insulated-Semiconductor) transistor using no oxide film maybe substituted. The same effect of the present invention is obtained.

According to the present invention, the LSI consolidating the logiccircuit and the SRAM circuit can reduce the power consumption at thetime of standby.

1. A semiconductor device comprising: a first memory cell; and a secondmemory cell, wherein each of said first memory cell and said secondmemory cell includes four N-channel type MIS transistors, and wherein agate-insulating film thickness of a first MIS transistor of said firstmemory cell is smaller than that of a first MIS transistor of saidsecond memory cell.
 2. The semiconductor device according to claim 1,further comprising: an input/output circuit having an MIS transistor;and a logic circuit having an MIS transistor, wherein thegate-insulating film thickness of said MIS transistor of saidinput/output circuit is larger than that of said first MIS transistor ofsaid first memory cell, and the gate-insulating film thickness of saidMIS transistor of said logic circuit is smaller than that of said firstMIS transistor of said second memory cell.
 3. The semiconductor deviceaccording to claim 1, further comprising: an input/output circuit; and alogic circuit wherein the gate-insulating film thickness of an MIStransistor in said logic circuit is equal to that of said first MIStransistor of said first memory cell, the gate-insulating film thicknessof a MIS transistor in said input/output circuit is equal to that ofsaid first MIS transistor of said second memory cell, said first memorycell additionally includes two P-channel type MIS transistors, saidsecond memory cell additionally includes two P-channel type MIStransistors, the four MIS transistors in each of said first memory celland said second memory cell are arranged such that the gates of a thirdMIS transistor and a fourth MIS transistor are connected to a wordline,the gate of said first MIS transistor is connected to said fourth MIStransistor, the drain thereof is connected to said third MIS transistor,the gate of a second MIS transistor is connected to said third MIStransistor, and the drain thereof is connected to said fourth MIStransistor.
 4. The semiconductor device according to claim 1, furthercomprising: an additional MIS transistor having a source-drain pathbetween an operating voltage supply point of said first memory cell anda power line, wherein said additional MIS transistor is controlled so asto be in the off state in a first state and to be in the on state in asecond state, and before said second state is changed to said firststate, information of said first memory cell is stored into said secondmemory cell.